Acute perinatal asphyxia, that is, hypoxia (insufficient oxygen saturation of fetal blood) during or close to birth, remains an important cause of neurological damage in form of hypoxic-ischemic encephalopathy (HIE) in newborn infants. It is seen in 2-9/1000 term infants and is followed by cerebral palsy (CP) and death in the severe cases. In a global perspective about 4 million newborn children die each year and about 23% are caused by acute perinatal asphyxia. Due to a lack of resources undeveloped countries are worse off, but as understood by the large numbers also in the western world this is a serious problem. Sweden can be seen as a representative country for the western world wherein asphyxia will occur in about 7/1000 term births leading to 2/1000 children being born with HIE. To prevent persisting damage caused by perinatal asphyxia it is important to detect hypoxia in a fetus as fast as possible upon its onset. Fast detection allows one to make decisions about whether to intervene at a stage where persisting damage has not occurred. The intervention substantially consists in bringing the infant out as quickly as possible by instrumental delivery, in particular by caesarean section.
Detection of acute perinatal asphyxia is presently done by monitoring of fetal heart rate followed by measurement of pH or lactate measurement in fetal scalp blood sampled through the vagina if an ominous fetal heart rate pattern is seen.
pH and lactate are indicators of metabolic acidosis caused by a switch to anaerobic metabolism in situation of insufficient oxygen supply. In an oxygen starved fetus pyruvate is metabolised to lactate and energy. At present measurement of pH is the golden standard. The fast determination of pH however requires about 35 μl of scalp blood, which is not easily obtained. Failure in the first determination is quite common (20%) as some studies indicate. Lactate is easier to measure since only 5 μl of blood is needed and the analysis can be carried out at bed side. Lactate analysis can be carried out within one minute and thus is sufficiently quick.
Lactate and pH are also indicators of acute asphyxia. As such Lactate and pH can provide an indication of a totally healthy fetus subjected to a sudden acute onset of hypoxia-ischemia during birth. A significant proportion of all infants developing hypoxic ischemic encephalopathy have had episodes of hypoxia-ischemia before entering the delivery phase. They are more vulnerable to hypoxia-ischemia during birth and do not respond in the same way as healthy foetuses and therefore the currently used methods are not sufficient for this group of patients.
A method for monitoring childbirth comprising the measurement of lactate in fluids, such as vaginal fluids, is disclosed in WO 2005/034762 A1. Preliminary results from a recent Swedish randomized study of pH and lactate in fetal blood at partum show that lactate, as an indicator of acidosis, is as good as pH. Neither lactate nor pH are ideal predictors of moderate/severe HIE: the sensitivity is only 67% for lactate and 50% for pH, whereas the specificity is about the same, 76% for lactate and 73% for pH. Also, the sensitivity and specificity in predicting acidosis in newborns are less than 70 percent for both lactate and pH. A recent Swedish report concludes that even in fetal monitoring by a combination of cardiotography (CTG) and STAN (analysis of the cardiographic ST segment) there is a risk of not detecting perinatal asphyxia of a kind that may result in encephalopathic damage (SBU Alert-rapport nr 2006-04).
Enzymes known to be elevated in newborn infants subjected to asphyxia during labor are LDH (lactate dehydrogenase), ALT (alanine aminotransferase) and AST (aspartate aminotransferase) also known as liver enzymes. LDH is found in most of the cells in the body, and it is considered an unspecific enzyme. Therefore it is infrequently used in clinical work. Previously LDH was used as a marker of myocardial damage but has now been replaced by more specific tests. AST and, in particular, ALT are more specific to liver damage. In a study on the effect of labor and delivery on plasma hepatic enzymes in the newborn of a group of low-risk Chinese women LDH (lactate dehydrogenase), ALT (alanine transaminase), AST (aspartate transaminase), GGT (γ-glutamyl transaminase) were determined and correlated to maternal and neonatal characteristics (Mongrelli M et al., J Obstet Gynaecol Res, 26(1): 61-63, 2000).
If a fetus is subjected to hypoxia during or close to birth the blood flow in its body will be redistributed from “less important organs” (kidneys, liver, fat and gut) in favor of the brain, heart and adrenals. This tends to damage cells in unprivileged organs. Cell damage results in a leakage of enzymes, which enter circulation. If the hypoxia is severe cells will die, and enzyme concentration increase even more in the blood. The rate of decline of LDH, AST and ALT (between 12-36 hours) makes it possible to detect cases more vulnerable to hypoxia-ischemia during delivery due to previous hypoxia started before the delivery. Hypoxia also affects the balance of electrolytes in the fetal body. One example is the influx of calcium from the blood in to the cells during hypoxia. It is shown that the concentration of serum ionized calcium is increased during hypoxia in a newborn animal model. Calcium also predicts outcome (brain damage or not) in human infants. Other enzymes and electrolytes of interest that are changed during hypoxia in newborn mammals are potassium (K+), magnesium (Mg2+), sodium (Na+), glucose, creatinine kinase (CK) and GGT.
In a global perspective about 4 million newborn children die each year and about 23% of these deaths are caused by acute perinatal asphyxia. The pathological mechanism of hypoxia-ischemia leading to injuries of the neurons in the brain is biphasic starting with the primary phase directly after birth. If an infant is successfully resuscitated this primary phase will be followed by a free interval that continues for hours. In two out of seven asphyxiated infants this free interval will be followed by a secondary energy depletion resulting in delayed cell death in the child's brain and a clinical picture with seizures (also known as HIE). The free interval offers a possibility to minimize the delayed cell death by hypothermia treatment (cooling the child's brain to 34.5° C.). Currently, however, there is no reliable means for predicting which child will develop HIE and therefore have benefits from hypothermia treatment.
In addition to during or close to birth, hypoxia is also a serious concern in many other medical conditions. For example, colorectal cancer is one of the most common tumors in both genders, the incidence of which is increasing every year. The current treatment involves a surgical procedure whereby the tumor is removed together with a radical part of the bowel. In the majority of these cases the distal and the proximal ends of the bowel are thereafter put together again. This is referred to as an anastomosis. During this procedure, arterial blood supply to the part of the bowel where the tumor is located is interrupted when the arterial vessels are cut. Post-operative complications due to leakage of the anastomosis can be anticipated in 7-10% of all operations. In such cases, the contents of the bowel will leak out in the abdomen and cause inflammation, peritonitis, sepsis and potentially death. The main reason for this complication is insufficient blood supply to the area for the anastomosis (e.g. hypoxia-ischemia) as a result of the extirpation of the vessels. The current solution is to simply re-operate. Undesirably, it is currently not possible, during surgery, to foresee if the anastomosis will leak or not.
Other areas where hypoxia is a major concern include vascular surgery and liver transplant surgery. For instance, a major factor determining morbidity and mortality in patients after liver transplantation therapy is preservation injury of the hepatic grafts (Leemaster 1997). LDH, AST and ALT leakage into the perfusate is a measure of loss of the membrane integrity of the liver cells (Kebis 2007).
One major shortcoming of prior devices and methods for determining hypoxia is that LDH can only be analyzed in plasma or serum. Further, they require a minimum of 150 microliters of whole blood for such a measurement. Such volume of blood is difficult to receive from small animals, specific tissue or the unborn child during birth. Another problem is that LDH is also present in red blood cells and haemolysis (rupture of red blood cells) will lead to false high values (beyond detection limit).
Thus, there is a need for a method of analyzing LDH, alone or together with electrolytes and liver enzymes, quickly and require only a small amount of blood for detection of hypoxia-ischemia. Additionally, there is a need for improvement in determining the oxygen supply status of a fetus at partum.